With grey cells firing and knitting needles clicking, Hercule Poirot and Miss Marple make their triumphant return to PBS in “Six by Agatha,” a half dozen whodunits by the greatest mystery author of all time. David Suchet (Henry VIII, The Way We Live Now) reprises his signature role of the Belgian private eye who cracks cases with the help of his “little grey cells,” and award-winning actress Julia McKenzie (Cranford) debuts as the newest grandmotherly sleuth.